Polyphenylene ether (PPE) resin compositions alone or in blends with other resins are widely used because of their excellent properties. Polyphenylene ether resin compositions and mixtures thereof with a polystyrene resin are described for example in European Patent Application No. 0,260,552. It is disclosed therein that very desirable improvements in impact resistance, heat resistance, mechanical strength, surface appearance, and moldability and flow properties are achieved if an additive comprising a silicone-based graft copolymer is used. In particular, there is used a modifier comprising a polyorganosiloxane rubber substrate onto which is subsequently grafted a stage or stages comprising two different vinyl monomers. Also relevant is BASF, U.K. Patent Specification No. 1,590,549, especially Example 7, which describes a composition comprising 90 wt. percent of poly-2,6-dimethyl-1,4-phenylene oxide with polystyrene (Noryl.RTM. 731, General Electric Company) and 10 wt. percent of a silicone rubber graft polymer.
A novel polyorganosiloxane/polyvinyl-based graft polymer has now been discovered to be extremely useful, for example, as an impact modifier for such thermoplastic resins. It also demonstrates surprising and unexpected ability to provide improved flame retardant compositions based on such resins, while maintaining other properties at desirable levels. As will be shown, PPE has a 1/16" UL flammability rating of V-1; adding a silicone graft copolymer to improve impact, e.g., EPO 0,260,552, increases impact but maintains the flammability rating of V-1.
In the present invention, the polyorganosiloxane rubber substrate of the prior art is replaced by a co-homopolymerized network(s) of polyorganosiloxane/ polyvinyl-based polymer(s) in a co-homopolymerization process. Polyorganosiloxane/polystyrene-based co-homopolymers are particularly preferred for use in the first stage of the graft polymers used in the present invention. The present invention uses graft polymers provided by subsequent graft polymerization of vinyl monomers (e.g. poly(methyl methacrylate), polystyrene or styrene/acrylonitrile copolymer) onto the polyorganosiloxane/polyvinyl substrate as described above.
This multi-stage graft polymer is usefully employed in the present invention as a superior modifier for thermoplastic molding compositions comprising polyphenylene ethers and mixtures thereof with polystyrene, and especially enhances the flame retardancy of such compositions. The molding compositions maintain many unique features of silicone rubber such as enhanced impact strength, particularly at low temperature, while maintaining other properties, such as weathering and thermal resistance. The flame-retardant embodiments are easily able to pass the stringent V-0 rating tests of Underwriter's Laboratory's Bulletin 94, while those of the prior art have difficulty in reaching the even less stringent V-1 rating.